Finally.. after lot of googling I found the solution. So, I'm sharing the same, just in case if anyone needs same thing in future
I changed the javascript
to below :
$(function() {
$("#file_upload").uploadify({
'formData' : {'fileName' : 'hello', 'fileNum' : 1},
'swf' : 'uploadify.swf',
'uploader' : '../upload',
'onUploadStart' : function(){
$("#file_upload").uploadify('settings','formData' ,{'fileNum': $('#fileNum').val()}); // to change value dynamically
}
});
});
and added following code inside upload servlet
if (item.isFormField()) {
System.out.println(item.getFieldName()); // field name
System.out.println(item.getString()); // and field value
// Here we can retreive all the Form Data
}